I have value 123
in binary system, this is 1111011, which is of length 7.
Is it correct to say 123 is 7 bit number?
Yes, it would be safe to say that it's a 7 bit number. But keep in mind that if you want to use this information in programming for instance, it would round up to 8 bits (or 1 byte), because there are no 7-bit numerical data types. So if you were to say 7-bit number to a bunch of programmers, that would just mean 8-bit and it'll be better to say 8-bit rather than 7-bit.